NASA’s TESS, Spitzer Missions Discover a World Orbiting a Unique Young Star


For more than a decade, astronomers have searched for planets orbiting AU Microscopii, a nearby star still surrounded by a disk of debris left over from its formation. Now scientists using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope report the discovery of a planet about as large as Neptune that circles the young star in just over a week.

NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope have found a young Neptune-size world orbiting AU Microscopii, a cool, nearby M-type dwarf star surrounded by a vast disk of debris. The discovery makes the system a touchstone for understanding how stars and planets form and evolve. Credits: NASA’s Goddard Space Flight Center Download high-resolution video and images from NASA’s Scientific Visualization StudioEn español

The system, known as AU Mic for short, provides a one-of-a-kind laboratory for studying how planets and their atmospheres form, evolve and interact with their stars.

“AU Mic is a young, nearby M dwarf star. It’s surrounded by a vast debris disk in which moving clumps of dust have been tracked, and now, thanks to TESS and Spitzer, it has a planet with a direct size measurement,” said Bryson Cale, a doctoral student at George Mason University in Fairfax, Virginia. “There is no other known system that checks all of these important boxes.”

The new planet, AU Mic b, is described in a paper co-authored by Cale and led by his advisor Peter Plavchan, an assistant professor of physics and astronomy at George Mason. Their report was published on Wednesday, June 24, in the journal Nature.

AU Mic b is featured in a new NASA poster available in English and Spanish, part of a Galaxy of Horrors series. The fun but informative series resulted from a collaboration of scientists and artists and was produced by NASA’s Exoplanet Exploration Program Office. 

Horror movie poster: AU Mic's Flares of Fury
Located less than 32 light-years from Earth, AU Microscopii is among the youngest planetary systems ever observed by astronomers, and its star throws vicious temper tantrums. You’ve heard of the “terrible twos”? Well, AU Mic is in the midst of its terrible 22 … millions! NASA celebrates this discovery with a new poster — based on real science — in its popular Galaxy of Horrors series. Credits: NASA-JPL/Caltech Download for free in English and Spanish.

AU Mic is a cool red dwarf star with an age estimated at 20 million to 30 million years, making it a stellar infant compared to our Sun, which is at least 150 times older. The star is so young that it primarily shines from the heat generated as its own gravity pulls it inward and compresses it. Less than 10% of the star’s energy comes from the fusion of hydrogen into helium in its core, the process that powers stars like our Sun.

The system is located 31.9 light-years away in the southern constellation Microscopium. It’s part of a nearby collection of stars called the Beta Pictoris Moving Group, which takes its name from a bigger, hotter A-type star that harbors two planets and is likewise surrounded by a debris disk.

Although the systems have the same age, their planets are markedly different. The planet AU Mic b almost hugs its star, completing an orbit every 8.5 days. It weighs less than 58 times Earth’s mass, placing it in the category of Neptune-like worlds. Beta Pictoris b and c, however, are both at least 50 times more massive than AU Mic b and take 21 and 3.3 years, respectively, to orbit their star.  

“We think AU Mic b formed far from the star and migrated inward to its current orbit, something that can happen as planets interact gravitationally with a gas disk or with other planets,” said co-author Thomas Barclay, an associate research scientist at the University of Maryland, Baltimore County and an associate project scientist for TESS at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “By contrast, Beta Pictoris b’s orbit doesn’t appear to have migrated much at all. The differences between these similarly aged systems can tell us a lot about how planets form and migrate.”

Detecting planets around stars like AU Mic poses a particular challenge. These stormy stars possess strong magnetic fields and can be covered with starspots — cooler, darker and highly magnetic regions akin to sunspots — that frequently erupt powerful stellar flares. Both the spots and their flares contribute to the star’s brightness changes.

In July and August 2018, when TESS was observing AU Mic, the star produced numerous flares, some of which were more powerful than the strongest flares ever recorded on the Sun. The team performed a detailed analysis to remove these effects from the TESS data.

When a planet crosses in front of its star from our perspective, an event called a transit, its passage causes a distinct dip in the star’s brightness. TESS monitors large swaths of the sky, called sectors, for 27 days at a time. During this long stare, the mission’s cameras regularly capture snapshots that allow scientists to track changes in stellar brightness.

Regular dips in a star’s brightness signal the possibility of a transiting planet. Usually, it takes at least two observed transits to recognize a planet’s presence.

“As luck would have it, the second of three TESS transits occurred when the spacecraft was near its closest point to Earth. At such times, TESS is not observing because it is busy downlinking all of the stored data,” said co-author Diana Dragomir, a research assistant professor at the University of New Mexico in Albuquerque. “To fill the gap, our team was granted observing time on Spitzer, which caught two additional transits in 2019 and enabled us to confirm the orbital period of AU Mic b.”

Spitzer was a multipurpose infrared observatory operating from 2003 until its decommissioning on Jan. 30, 2020. The mission proved especially adept at detecting and studying exoplanets around cool stars. Spitzer returned the AU Mic observations during its final year.

Because the amount of light blocked by a transit depends on the planet’s size and orbital distance, the TESS and Spitzer transits provide a direct measure of AU Mic b’s size. Analysis of these measurements show that the planet is about 8% larger than Neptune.

Observations from instruments on ground-based telescopes provide upper limits for the planet’s mass. As a planet orbits, its gravity tugs on its host star, which moves slightly in response. Sensitive instruments on large telescopes can detect the star’s radial velocity, its motion to-and-fro along our line of sight. Combining observations from the W. M. Keck Observatory and NASA’s InfraRed Telescope Facility in Hawaii and the European Southern Observatory in Chile, the team concluded that AU Mic b has a mass smaller than 58 Earths.

This discovery shows the power of TESS to provide new insights into well-studied stars like AU Mic, where more planets may be waiting to be found.

“There is an additional candidate transit event seen in the TESS data, and TESS will hopefully revisit AU Mic later this year in its extended mission,” Plavchan said. “We are continuing to monitor the star with precise radial velocity measurements, so stay tuned.”

For decades, AU Mic has intrigued astronomers as a possible home for planets thanks to its proximity, youth and bright debris disk. Now that TESS and Spitzer have found one there, the story comes full circle. AU Mic is a touchstone system, a nearby laboratory for understanding the formation and evolution of stars and planets that will be studied for decades to come.

TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.

NASA’s Jet Propulsion Laboratory (JPL) in Southern California managed the Spitzer mission for the agency’s Science Mission Directorate in Washington. Spitzer science data continue to be analyzed by the science community via the Spitzer data archive located at the Infrared Science Archive housed at IPAC at Caltech in Pasadena. Science operations were conducted at the Spitzer Science Center at Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. Caltech manages JPL for NASA.

Banner image: This illustration depicts one interpretation of the planet AU Mic b and its young red dwarf host star. The system lies about 32 light-years away in the southern constellation Microsopium. Credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA)By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.

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Ron Long
June 26, 2020 10:50 am

Awesome science! But we don’t have to send Captain Kirk to AU Mic b because for sure he would say “beam me up Scotty, there’s no intelligent life on this planet”. Maybe life as advanced as pond scum?

Reply to  Ron Long
June 26, 2020 12:20 pm

Amazing pseudo science. They can only detect the presence of a planet by intensity fluctuations but claim to know there is a “ring of dust”. I wonder how they detected that, or if they just made it up because that’s how they believe planets are formed.

John W Braue
Reply to  Greg
June 26, 2020 3:09 pm

Dust rings are found by excess infrared emissions.

Reply to  John W Braue
June 26, 2020 3:23 pm

They are inferred from signals of assumed/asserted fidelity, and patterns in a data collection. The construction of planets from “dust rings” follows the same logical progression.

Reply to  Greg
June 27, 2020 8:16 am
June 26, 2020 12:09 pm

Pick your poison.

June 26, 2020 2:34 pm

My wondering two things.
Would the one Neptune like planet, help form other planets. Or Jupiter thought perturb space rocks making our NEO population young. Though Jupiter also blamed for stopping planet formation between Mars and Jupiter.
Second will this violent sun explode so as to remove the hydrogen and helium gases, and leave the planet with just heavier gases.
Or could Neptune type become like Venus.
And such explosion, thinking may occur when sun, starts emitting more light for nuclear reaction rather than from it primordial heat.

Paul Johnson
June 26, 2020 3:26 pm

Interesting article, but in a universe containing “billion and billions” of stars, isn’t it a bit far-fetched to call any one star “unique”? That is, unless one considers every star unique.

Dudley Horscroft
June 26, 2020 8:19 pm

“We think AU Mic b formed far from the star and migrated inward to its current orbit, something that can happen as planets interact gravitationally with a gas disk or with other planets,” said co-author Thomas Barclay,”

Thinks – if the planet started way out and then migrated inwards, and it was a gas disc (note correct spelling) that slid it down the gravity funnel, then the gas disc must have been pretty dense. On the way down it must have collected a lot of the gas as it cleared a path. Perhaps it started out as large as Mercury – that would have done it, now as large as Neptune.

On the other hand, if it was originally on a near circular orbit somewhere in the local Oort cloud, then it would have needed a near collision with a similar sized (or more likely much larger) planet to send it into an ellipse with a vertex near its sun, and then another near sized planet to convert this ellipse into a near circular (one presumes) stable orbit. What are the odds of these three planets being there, and just managing to arrange such a change?

Or is it more likely that it formed close to the sun?

June 26, 2020 8:27 pm

So if the star can be tugged of a straight line between here and there as a response to the gravitational attraction of a major planet, then there must also be a barycenter for the system. Presumably the center of mass of the star orbits the barycentre. Wonder if this has an effect on the total radiant output of this star? Is anyone monitoring solar systems with a number of particularly big planets to see if the effect of such motions have any effect on solar output?

Rainer Bensch
June 27, 2020 4:08 am

Oh, stopped reading at
It weighs less than 58 times Earth’s mass

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